remove the subscriber cache by going to the subscriber and removing all directories under:

/PortalServer/wcm/ilwwcm/system/subscriber

(This folder is used by the syndicator to store content items retrieved from the subscriber prior to being saved to the JCR. If you look at this folder during a syndication, you will see it fill with files as they are being retrieved by the subscriber. You will also see this folder start to empty as items are moved from this temporay location and stored in the JCR

Lastly re-create syndicator and subscriber

Other things you may want to check

Make sure you have the latest fix pack

If your syndicator or subscriber is clustered then also:

Ensure that you have load balancer names/addresses + ports defined in WCMConfigService.properties, syndicators and subscribers.

Ensure that you have disabled syndicator and subscriber cache using cachespec.xml